1. Department of Mathematics, University of South Carolina, Columbia, SC 29208, USA
2. Department of Mathematics, University of North Carolina, Chapel Hill, NC 29599, USA
3. School of Materials Science and Engineering, Nankai University, Tianjin, P. R. China
4. Beijing Computational Science Research Center, Beijing, P. R. China
5. Department of Mathematics, Purdue University, West Lafayette, IN 47906, USA